Am Samstag 17 April 2010 22:01:23 schrieb Limestraël:
> Yes! Sorry, I forgot a bit:
> Binary types are automatically made instances of
> Binarizable/Unbinarizable (that's my line 16):
>
> instance (Binary a) => Binarizable a a where
> toBinary = id
>
> instance (Binary a, Monad m) => Unbinarizable a a m where
> fromBinary = return
>
And that is your problem.
The compiler only looks at the context (Binary a) *after* it has chosen an
instance.
When somewhere in the code it encounters "toBinary x", it looks for an
instance declaration "instance Binarizable a b where" which matches x's
type. Since you have "instance Binarizable a a", you have a matching
instance and that is selected. *Now* the compiler looks at the context and
barfs if x's type is not an instance of Binary.
>
> To me, the functional dependency in:
> class (Binary b) => Binarizable a b | a -> b
> meant that for each a, there only one type b that can match.
Yes, that's what the functional dependency says.
>
> That's what I want: for every Binary type 'a', the matching Binary is
> also 'a'
And that instance says "for every type 'a', the matching type is also 'a',
and furthermore, 'a' is an instance of Binary".
Contexts on a class and functional dependencies don't work as one would
naively expect.
> And for GameObject, the sole matching type is String.
> In other words, GameObject implies String.
> I would have undestood the error if GameObject was also an instance of
> Binary (then the two instances would match), but it's not the case...
The context isn't considered until after matching.
>
> Is my FunDep wrong?
At least, the FunDep plus the generic instance is not what you want.
Probably, what you want can be done with some type wizardry, but I don't
know how.
Perhaps the following will work:
{-# LANGUAGE OverlappingInstances, TypeFamilies, MultiParamTypeClasses #-}
class Binarizable a where
type ToBin a
toBinary :: a -> ToBin a
class (Monad m) => Unbinarizable a m where
type FromBin a
fromBinary :: FromBin a -> m a
instance Binarizable GameObject where
type ToBin GameObject = String
toBinary g = ...
instance (Binary a) => Binarizable a where
type ToBin a = a
toBinary x = x
instance (MonadReader [GameObject] m) => Unbinarizable GameObject m where
type FromBin GameObject = String
fromBinary s = ...
instance (Monad m, Binary a) => Unbinarizable a m where
type FromBin a = a
fromBinary x = return x
With OverlappingInstances, the most specific match is chosen, so for
GameObjects, the special instance is selected.
>
> I done this especially because I didn't wanted to declare each type one
> by one instance of Binarizable,
> Haskell type system normally enables me to automatically define a Binary
> as an instance of Binarizable.
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